This invention relates to a process by which the nutritional value of protein obtained from green plants is enhanced. More specifically, the present invention relates to a process whereby the protein recovered from leafy green plants consists of a greater percentage of the more nutritious cytoplasmic protein, as opposed to the less nutritious chloroplastic protein.
In the recovery of protein from green plants, normally a protein-containing juice is liberated from the plants by pressing, or the like. This juice contains two forms of protein, i.e., cytoplasmic protein and chloroplastic protein. The cytoplasmic protein is associated with the cytoplasm of a plant, whereas the chloroplastic protein is associated with chloroplasts of the plant. The cytoplasm is a liquid contained within the cell walls of the plant and the cytoplasmic protein is in solution in the cytoplasm. The chloroplasts are solid particles dispersed throughout a plant cell and the chloroplastic protein is physically associated with those chloroplasts. Also contained with the juice are sugars, salts, vitamins, pigments and other plant materials.
After the protein-containing juice has been liberated from the plants, the protein is separated from the juice. This may be accomplished by heating the juice to a temperature of 70.degree. C. or more by adjusting the pH of the juice to a value below 5.0 or by a combination of these steps. In either case, most of the protein will coagulate and precipitate from the juice. Then the protein coagulum, or curd, may be isolated by conventional means such as decantation, filtration or centrifugation.
One such prior art method for recovering protein from green leafy plants is the wet fractionation process described in U.S. Pat. No. 3,775,133, to William R. Batley, Jr., one of the inventors herein. The disclosure of that patent is incorporated herein by reference.
Although the wet fractionation process of U.S. Pat. No. 3,775,133 provided a method for increasing the protein recovery from leafy green plants, it produces a product having a cytoplasmic protein content of less than about 45%. Other commercial prior art processes provide even less total protein recovery and no better cytoplasmic protein content.
In some cases it is desirable to have a protein product which is high in cytoplasmic protein. The desirability of cytoplasmic protein stems from the fact that it is more easily digested under the acid conditions of the stomach than is chloroplastic protein, and is therefore more efficient in some uses.
An indicator of the nutritional value of protein is its protein efficiency ratio, or PER, which is defined as the grams in weight gain of test animals per gram of protein eaten. Laboratory work has shown that the protein efficiency ratio of chloroplastic protein is between about 0.5 and 0.8. On the other hand, the PER of cytoplasmic protein is about 2.5. It is recommended that protein to be used as nutritional supplements have a protein efficiency ratio approaching that of casein, a phosphoprotein of milk, which has a PER of about 2.5.
Much attention has been directed toward solving the low nutritional value problem of green plant protein. Although initially such protein was not even a very suitable feed for non-ruminant animals, this attention has resulted in substantial improvements in the digestibility, protein efficiency ratio and other measures of the nutritional value of green plant protein. For instance, it is now known that the temperature at which the product is dried is of utmost importance. That is, if the product is dried at 40.degree. C., rather than 100.degree. C., the nutritional value thereof is approximately doubled. Also, the desirability of rapidly processing the green plants once harvested and of washing the product before drying is now known. As a result of such developments, currently produced green plant protein generally has a protein efficiency ratio of between 1.2 and 1.5.
Recently, to obtain green plant protein with an even better nutritional value, two approaches have been taken. One involves the supplementation of the green plant protein with certain amino acids, while the other involves the fractional separation of the protein into a chloroplastic portion and a cytoplasmic portion.
In the first approach, methionine, sometimes along with lysine and leucine, is added in small amounts to a green plant protein product. Since these are the limiting essential amino acids, their addition has an ameliorative effect on the nutritional value of the protein.
In the second approach, the chloroplastic protein, along with the chloroplasts with which it is associated, is separated from the protein-containing juice by either high speed centrifugation or by a process based on the fact that the chloroplasts and chloroplastic protein will coagulate and separate from the other plant components in the juice at a lower temperature than the cytoplasmic protein. Such a separation allows for the eventual protein product to be comprised of substantially all high nutritional value cytoplasmic protein.
However, it should be noted that by utilizing fractional separation in the aforementioned manner, the protein product yield is dramatically reduced. This is due to the fact that for most green plants, cytoplasmic protein accounts for only about 30-45% of the total protein recoverable from the plants by previously known commercial methods, and the cytoplasmic protein fraction is substantially the only protein being recovered.